A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol, e.g., a methanol (feed) composition, with carbon monoxide. The catalyst may contain rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium comprises acetic acid, methyl acetate, water, methyl iodide and the catalyst. The methanol and the carbon monoxide come into contact in the reaction medium and react with one another to form crude acetic acid. Conventional commercial processes for the carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entireties of which are incorporated herein by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entirety of which is incorporated herein by reference.
The carbonylation reaction forms reaction by-products such as water, (residual) methyl iodide, and carbonyl impurities, e.g., acetaldehyde. These by-products are typically separated and/or recycled within the system. Hydrogen iodide is another undesirable component that may be present in the reaction medium. The presence of hydrogen iodide in the reaction system is particularly troublesome because it acts as a corrosion agent that may cause metallurgical problems throughout the reaction and separation zones. Thus, the need exists for reducing the formation of hydrogen iodide in carbonylation processes.
US Patent Application Publication No. 2013/0116470 discloses a production process of acetic acid comprising a reaction step for continuously allowing at least one member selected from the group consisting of methanol, dimethyl ether, and methyl acetate to react with carbon monoxide in a catalyst system comprising a rhodium catalyst, an iodide salt, and methyl iodide in the presence of acetic acid and water in a plant compromising a reactor; a flasher; and a distillation column; wherein part of the vaporized stream is introduced into a heat exchanger. The process achieves a production of acetic acid with a high purity in a resource-saving and energy-saving equipment by efficiently removing a reaction heat even in a large-sized plant.
U.S. Pat. No. 8,318,977 discloses a methanol carbonylation system including an absorber tower adapted for receiving a vent gas stream and removing methyl iodide therefrom with a scrubber solvent, the absorber tower being coupled to first and second extractant sources, which are capable of supplying different first and second extractants. A switching system including valves alternatively provides first or second extractants to the absorber tower and returns the used solvent and absorbed material to the carbonylation system to accommodate different operating modes. This reference does not mention hydrogen iodide.
While the above-described processes provide general processes for purifying a crude acetic acid product, these processes fail to specifically address the separation of hydrogen iodide from the crude product. The need exists for improved processes for producing acetic acid that provide for separation and removal of hydrogen iodide from a crude acetic acid product.